Combine in a saucepan the egg yolks, butter or ghee, 2 T sweetener, lemon zest, and lemon juice.
Over medium heat, stir the mixture until it begins to thicken.
When the curd coats a spoon and begins to pull away from the bottom and sides of the pan with stirring, remove from heat.
Strain out lemon zest and lumps with a jelly strainer to leave a smooth curd.
Pour the curd into 2 individual ramekins or an 8-ounce baking dish.
Refrigerate for at least 3 hours, until completely cooled.
Just before serving, use a hand mixer or stand mixer to whip two to three of the reserved egg whites with 1 teaspoon of sweetener and the cream of tartar. Beat until stiff peaks form.
Spoon or pipe beaten egg whites onto the top of the lemon curd.
Place under a low broiler until meringue is browned. Watch carefully, as this goes very quickly!
Serve immediately, or hide in the schoolroom and eat both servings yourself.